Even a set of the less expensive 6x-8 400 heads would be a good upgrade, normally somewhere around 101 to 105cc chamber volume. With a cleanup cut they'd put your compression at a nice 9:1 ratio, plenty of compression to add some pep and be able run whatever premium pump gas you might find and would tolerate regular in a pinch if premium isn't available.
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1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/2.56
2002 WS6 Convertible LS1/4L60E/3.23
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